Some children are born missing half their heart. Known as single ventricle heart disease, it is the leading cause of death in children less than one year of age from any structural birth defect. One common anatomic variant is Hypoplastic Left Heart Syndrome. Until recently this condition was not compatible with survival. Beginning in the 1970's, developments in the surgical treatment of single ventricle heart disease have resulted in a means of not only survival, but also reasonable quality of life for survivors at least into early adulthood. Current therapy includes a series of 3 staged open heart procedures. While these procedures offer hope for survival, they remain problematic and notorious for instability and mortality. The staged surgical reconstruction of the circulatory system culminates in a univentricular Fontan circulation, eponymous with Dr. Francis Fontan who first described the repair in 1971.
In a univentricular Fontan circulation, the single ventricle (pumping chamber) is committed to provide blood flow to the body. Opposed to a normal 2-ventricle circulation, however, blood flow through the lungs is not supported by a ventricular power source; it should rather flow through the lungs passively. Therefore, the motive force for blood flow through the lungs rests upon systemic venous pressure alone. As a consequence, systemic venous pressure is markedly elevated and systemic venous return is significantly altered. This sets up a new set of hemodynamic problems, described by de Leval as the Fontan paradox, in which elevated systemic venous pressure coexists with relative pulmonary arterial hypotension. Preload to the single ventricle is reduced, as well as cardiac output. Patients with a univentricular Fontan circulation are therefore at high risk for late Fontan failure and attrition.
The late consequences of this circulatory arrangement are now an emerging public health concern. Thousands of patients who survive Fontan palliation are expected to present with Fontan failure. The insidious complications of chronically elevated systemic venous pressure include hepatic and gut dysfunction, protein losing enteropathy, leg swelling, and collection of fluid in the abdominal and chest cavities. The insidious complications of chronically reduced preload include late ventricular diastolic dysfunction, and poor systemic tissue perfusion. Targeted medical therapeutic options for Fontan failure do not exist. For example, while diuretic therapy may improve symptoms of increased tissue/organ edema, it does so at the expense of circulating blood volume which is helpful to Fontan circulatory homeostasis. Similarly, although the use of inotropic support may improve myocardial contractility, this is of marginal impact in an insufficiently filled ventricle. Heart transplantation is a poor end-stage option: Transplantation carries morbidity of its own, and the donor pool is limited. Few patients will ultimately be candidates or receive a donor organ for transplantation.
The development of a permanent right-sided circulatory support device directly addresses the Fontan paradox and will improve late quality of life and outcomes for those born with single functional ventricle. One aspect of some embodiments has been to include power sources to support the univentricular Fontan circulation. The placement of a power source at the level of the total cavopulmonary connection effectively empowers the univentricular Fontan circulation by placing a right ventricle equivalent back into a circulation that lacks one. By simultaneously reducing systemic venous pressure and improving ventricular preload, normal 2-ventricle physiology can be effectively restored.
Prior applications of existing blood pump technology have been contemplated to address the problem of powering the Fontan circulation. These have consisted primarily of applying intravascular unidirectional axial flow pumps to augment Fontan flow. The concept of cavopulmonary assist was introduced in 2003 with the simultaneous use of 2 unidirectional axial flow pumps (Rodefeld et al, Ann Thoracic Surg). This has limitations, however, as one-way flow devices will cause undesirable back-pressure elevation in the opposing vena caval territory. Other groups have followed with modifications of axial flow pump designs intended to operate in the low-pressure systemic venous circulation. This has also included a modification of the preferred TCPC Fontan venous pathway to a 3-way pathway so that the pathway better accommodates a unidirectional pump in a common unidirectional outflow limb. Although in theory this is possible, the 3-way vascular configuration is not the preferred hemodynamic pathway in an unsupported Fontan circulation.
What follows are various improvements in the field of non-positive displacement circulatory pumps that overcome some of the disadvantages of existing systems.